1. Technical Field
The present application relates to a nitride-based semiconductor light-emitting device which has polarization characteristics.
2. Description of the Related Art
A nitride semiconductor containing nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device, because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors have been researched and developed particularly extensively. As a result, blue light-emitting diodes (LEDs), green LEDs, and blue semiconductor laser diodes in which gallium nitride-based compound semiconductors are used have already been used in actual products.
Hereinafter, the gallium nitride-based compound semiconductors are referred to as nitride semiconductors. The nitride semiconductors include a compound semiconductor in which some or all of gallium (Ga) atoms are replaced with at least one of aluminum (Al) and indium (In) atoms. Therefore, the nitride semiconductors are represented by compositional formula AlxGayInzN (0≦x, y, z≦1, x+y+z=1).
By replacing Ga atoms with Al atoms, the bandgap can be greater than that of GaN. By replacing Ga atoms with In atoms, the bandgap can be smaller than that of GaN. This enables not only emission of short-wave light, such as blue light or green light, but also emission of orange light or red light. Because of such a feature, a nitride-based semiconductor light-emitting device has been expected to be applied to image display devices and lighting devices.
The nitride semiconductor has a wurtzite crystal structure. FIGS. 1A, 1B, and 1C show planes of a wurtzite crystal structure using four characters (hexagonal indices). In a four-character expression, crystal planes and orientations are expressed using primitive vectors of a1, a2, a3, and c. The primitive vector c runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. FIG. 1A shows c-plane as well as a-plane and m-plane. The c-plane, the a-plane, and the m-plane are perpendicular to one another. FIG. 1B shows r-plane. FIG. 1C shows (11-22) plane.
FIG. 2A shows a ball-and-stick model of the crystal structure of the nitride semiconductor. FIG. 2B shows an atomic arrangement near an m-plane surface, which is observed from the a-axis direction that is perpendicular to the a-plane, i.e., the [11-20] direction. The m-plane is perpendicular to the drawing sheet of FIG. 2B. FIG. 2C shows an atomic arrangement at a +c-plane surface, which is observed from the m-axis direction. The c-plane is perpendicular to the drawing sheet of FIG. 2C. As seen from FIG. 2B, N atoms and Ga atoms reside at a plane which is parallel to the m-plane. On the other hand, as seen from FIG. 2C, the c-plane includes layers in which only Ga atoms reside and layers in which only N atoms reside.
In general, in fabricating a semiconductor device using nitride semiconductors, a c-plane substrate, i.e., a substrate which has a (0001)-plane principal surface, is used as a substrate on which nitride semiconductor crystals are to be grown. In this case, due to the arrangement of Ga atoms and N atoms, spontaneous electrical polarization is produced in the c-axis direction in the nitride semiconductor. That is why the c-plane is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated along the c-axis direction in the InGaN quantum well in the active layer of the nitride-based semiconductor light-emitting device. This electric field causes some positional deviation in the distributions of electrons and holes in the active layer, so that the internal quantum yield decreases due to the quantum confinement Stark effect of carriers.
Thus, it has been proposed that a substrate of which the principal surface is a so-called “non-polar plane”, such as m-plane or a-plane, or a so-called “semi-polar plane”, such as −r plane or (11-22) plane, be used. As shown in FIG. 1, the m-planes in the wurtzite crystal structure are parallel to the c-axis and are six equivalent planes which intersect with the c-plane at right angles. For example, in FIG. 1, the (1-100) plane that is perpendicular to the [1-100] direction is the m-plane. The other m-planes which are equivalent to the (1-100) plane include (−1010) plane, (10-10) plane, (−1100) plane, (01-10) plane, and (0-110) plane. Here, “-” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar”.
On the m-plane, as shown in FIG. 2B, Ga atoms and N atoms are on the same atomic-plane. For that reason, no electrical polarization will be produced perpendicularly to the m-plane. Therefore, if a light-emitting device is fabricated using a semiconductor multilayer structure which has been formed on the m-plane, no piezoelectric field will be generated in the active layer, thus overcoming the problem that the internal quantum yield decreases due to the quantum confinement Stark effect of carriers. This also applies to the a-plane that is one of the other non-polar planes than the m-plane. Also, similar effects can be achieved even in the case of a so-called semi-polar plane, such as −r plane or (11-22) plane.
Further, a nitride-based semiconductor light-emitting device including an active layer which is formed on the m-plane or a-plane or the r-plane or (11-22) plane has a polarization characteristic which is attributed to the structure of its valence band. Light which is emitted in the c-axis direction from an active layer of which principal surface is a c-plane is not polarized light. However, polarized light can be extracted from an active layer of which principal surface is inclined with respect to the c-plane.
Japanese Laid-Open Patent Publication No. 2008-305971 discloses, as a solution to improve light extraction with polarization being maintained in a Group III nitride-based semiconductor light-emitting device of which principal surface is a non-polar plane or a semi-polar plane, a configuration in which striped grooves which have a sawtooth shape are provided across a light extraction surface of the nitride-based semiconductor light-emitting device. The extending direction of the striped grooves is perpendicular to the polarization direction.
Applied Physics Express 2 (2009) 031002 discloses a configuration in which an m-plane GaN layer of excellent crystallinity is provided on an a-plane sapphire which has striped recesses and elevations. The extending direction of the striped recesses and elevations corresponds to the m-axis direction of the sapphire, i.e., the a-axis direction of the GaN.
Japanese Laid-Open Patent Publication No. 2008-109098 proposes a light-emitting diode device which is configured so as to reduce the difference in emission intensity which is attributed to the difference in in-plane azimuth angle of a nitride-based semiconductor light-emitting device. In the fifth embodiment of Japanese Laid-Open Patent Publication No. 2008-109098, the light emission surface of a package is configured such that the direction of light is changed to a direction of an azimuth angle in which the emission intensity is small so as to reduce the difference in intensity of light emitted from the package which is attributed to the difference in in-plane azimuth angle of the chip placing surface.